This invention is directed to a cathodic electrocoating composition and in particular to a cathodic electrocoating composition containing a blocked polyisocyanate cross-linking agent which significantly reduces bake-off loss occurring from the coating film during cure.
The coating of electrically conductive substrates by an electrodeposition process, also called an electrocoating process, is a well known and important industrial process. Electrodeposition of primers to metal automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of a coating composition of an aqueous emulsion of film forming polymer and the article acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the coating composition, until a coating of a desired thickness is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Film forming resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art. These resins typically are made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through a reaction of the resin with an amine compound. These resins are blended with a crosslinking agent usually a blocked polyisocyanate and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives such as a catalyst to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics of the tank, the immersion time, and the like.
The resulting coated article is removed from the bath and is rinsed with deionized water. The coating on the article is cured typically in an oven at sufficient temperature to form a crosslinked finish on the article. The presence of the catalyst enhances the crosslinking of the finish.
Cathodic electrocoating compositions, resin compositions, coating baths and cathodic electrodeposition processes are disclosed in Jarabek et al U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. Pat. No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat. No. 4,137,140 issued Jan. 30, 1979 and Wismer et al U.S. Pat. No. 4,468,307 issued Aug. 25, 1984.
One disadvantage associated with conventional electrocoating compositions containing blocked isocyanate crosslinking agents is that they suffer elimination losses during baking, known as bake-off losses, principally due to volatilization of blocking agents. The standard blocking agents are typically high molecular weight alkyl alcohols or ether alcohols, which produce significant film weight losses during the baking process, which makes it necessary to purify the exhaust air discharged from the oven and constitutes an unwanted loss in resin solids. In addition, the volatile blocking agents released during cure can cause other deleterious effects on various coating properties, e.g., a rough surface. U.S. Pat. No. 4,615,779 to McCollum et al issued Oct. 7, 1986 shows the use of lower molecular weight alcohol blocking agents for the polyisocyanate to reduce weight loss when the film is heated to cure. Such blocking agents, however, can produce undesirable film defects. Therefore, there is still a need to find new blocked isocyanate cross-linking agents for cathodic electrocoating compositions that provide reduced bake-off loss, while maintaining the desired coating properties.
An improved aqueous cathodic electrocoating composition having a film forming binder of an epoxy-amine adduct, a blocked polyisocyanate crosslinking agent and an organic or inorganic acid as the neutralizing agent for the epoxy-amine adduct; wherein the improvement is the use of a blocked polyisocyanate crosslinking agent that has at least one isocyanate group blocked with a hydroxy-functional cyclic carbonate compound for reduced bake-off loss on cure.